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Erosion behavior of SiC–WC composites
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6829–6839 www.elsevier.com/locate/ceramint
Erosion behavior of SiC–WC composites Sandan Kumar Sharmaa, B. Venkata Manoj Kumara,n, Kwang-Young Limb, Young-Wook Kimb, S.K. Natha b
a Department of Metallurgical and Materials Engineering, Indian Institute of Technology (IIT) Roorkee, Roorkee, India Functional Ceramics Laboratory, Department of Materials Science and Engineering, the University of Seoul, Seoul, Republic of Korea
Received 6 October 2013; received in revised form 29 November 2013; accepted 30 November 2013 Available online 10 December 2013
Abstract Dense silicon carbide (SiC) ceramics were prepared with 0, 10, 30 or 50 wt% WC particles by hot pressing powder mixtures of SiC, WC and oxide additives at 1800 1C for 1 h under a pressure of 40 MPa in an Ar atmosphere. Effects of alumina or SiC erodent particles and the WC content on the erosion performance of sintered SiC–WC composites were assessed. Microstructures of the sintered composites consisted of WC particles distributed in the equi-axed grain structure of SiC. Fracture surfaces showed a mixed mode of fracture, with a large extent of transgranular fracture observed in SiC ceramics prepared with 30 wt% WC. Crack bridging by WC enhanced toughening of the SiC ceramics. A maximum fracture toughness of 6.7 MPa*m1/2 was observed for the SiC ceramics with 50 wt% WC, whereas a high hardness of 26 GPa was obtained for the SiC ceramics with 30 wt% WC. When eroded at normal incidence, two orders of magnitude less erosion occurred when SiC–WC composites were eroded by alumina particles than that eroded by SiC particles. The erosion rate of the composites increased with increasing angle of SiC particle impingement from 301 to 901, and decreased with WC reinforcement up to 30 wt%. A minimum erosion wear rate of 6.6 mm3/kg was obtained for SiC–30 wt% WC composites. Effects of mechanical properties and microstructure on erosion of the sintered SiC–WC composites are discussed, and the dominant wear mechanisms are also elucidated. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Hot pressing; B. Composites; Silicon carbide; Tungsten carbide; Erosion
1. Introduction Erosion causes major material loss in several structural applications such as gas turbine parts, burner parts, cutting tools, and fluidized bed combustion systems [1]. Owing to their unique combination of superior properties such as high hardness, high temperature strength and excellent resistance to wear and corrosion, silicon carbide (SiC) ceramics are preferred for several tribological applications [2–4]. Liquid phase sintering using different additives like metal oxides, Al–B–C, and AlN–metal oxides has been preferred for the preparation of dense SiC ceramics with a tailored microstructure [4–6]. The mechanical properties were reported to improve by the incorporation of Si3N4 [7], TiC [8] or TiB2 [9] particles in the SiC ceramic matrix. n
Corresponding author. Tel.: þ91 1332 285420; fax: þ 91 1332 285243. E-mail address: [email protected] (B.V.M. Kumar).
Extensive research has been carried out in understanding erosion wear behavior of SiC ceramics and their composites [10–20]. Solid particle erosion of SiC ceramics and their composites is reported to occur generally by brittle fracture as a result of lateral and radial cracking [11–15]. Routbort et al. found that the morphology of the eroded surface of SiC ceramics varied with an increase in the size of the erodent particles [13]. The erosion wear of SiC ceramics is reported to not be in agreement with the predicted erosion wear rates because of the microstructural effects [16–18]. Jianxin et al. [19] studied erosion wear behaviors of SiC/(W,Ti)C laminated ceramic nozzles produced by hot pressing in dry sand blasting processes and reported that the laminated ceramic nozzles were superior to the homologous stress-free ceramic nozzle due to the formation of compressive residual stresses in the nozzle entry region. Kim and Park [15] reported that the erosion rates of hot-pressed monolithic SiC ceramics and SiC–TiB2 composites did not increase monotonically with increasing particle
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.146
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Table 1 Sample designation, batch composition, and sintered density of the investigated powder mixtures. Sample designation
SW0 SW10 SW30 SW50
Sintered density (g/cc)
Batch composition (wt%) β-SiC
α-SiC
WC
Al2O3
Y2O3
CaO
94.05 84.15 64.35 44.55
0.95 0.85 0.65 0.45
0 10 30 50
3.5 3.5 3.5 3.5
1.0 1.0 1.0 1.0
0.879 0.879 0.879 0.879
size. Rather, the erosion rate was higher for the SiC–TiB2 composite with higher fracture toughness but had a lower hardness compared to monolithic SiC, indicating that the erosion of these materials is largely controlled by the plastic deformation [15]. Large erodent particle size and elevated temperatures are reported to lead to lower erosion rates for SiC–TiB2 composites when compared against SiC ceramics [20]. A new combination that has received much less attention is the incorporation of tungsten carbide (WC) in SiC ceramics. Improved wear resistance is expected by adding tough and strong WC in the hard SiC matrix. Powder metallurgy route is suitable for manufacturing such composites. Pang and Li prepared porous SiC–WC composites by solid reaction of Si and WC for applications that involve low thermal expansion [21]. Zhang et al. prepared dense and strong SiC–Si composites reinforced by WC particle by hot pressing [22]. To the best of our knowledge, no report is available in the open literature on hot pressing of SiC and WC powders to prepare dense SiC–WC composites for their use in tribological applications. In the present work, SiC ceramic composites with varying WC content (0–50 wt%) were prepared by hot pressing technique. The erosion behavior of the sintered composites was estimated using alumina and SiC particles at normal incidence. Effects of angle of impingement (301, 601 and 901) of SiC particles and WC reinforcement on the erosion performance were particularly assessed. The dominant material removal mechanisms in the selected erosion conditions were elucidated.
2. Experimental procedure 2.1. Material preparation Commercially available β-SiC (BF-17, Hermann C. Starck, Germany), α-SiC (A1, Showa Denko, Japan), WC (High Purity Chemicals, Saitama Pre., Japan), metal oxides Al2O3, (AKP30, Sumitomo Chemical Co., Tokyo, Japan), Y2O3, (High Purity Chemicals, Saitama Pre., Japan) and CaO (High Purity Chemicals, Saitama Pre., Japan) were used as starting powders. Powder mixtures containing 0, 30, and 50 wt% WC were prepared. The respective powder batches were milled in ethanol for 24 h using SiC grinding balls. The milled slurry was dried, sieved, and hot-pressed at 1800 1C for 1 h under
3.23 3.51 4.31 5.45
Fig. 1. Schematic diagram of erosion set-up.
a pressure of 40 MPa in Ar atmosphere to obtain sintered samples of 30 mm in diameter and 4 mm in thickness. The details of sample designation, batch composition and additive systems are given in Table 1. 2.2. Characterization of sintered composites The volume and weight of the sintered specimens were measured and the bulk density was estimated. For calculating theoretical densities by rule of mixtures, the following theoretical densities (in g/cc) of each constituent are considered: β-SiC 3.216, WC 15.669, Al2O3 3.987, Y2O3 5.032, and CaO 3.374. Phase analysis of the sintered specimens was performed using X-ray diffractometry (XRD, D8 Discover, Bruker AXS GmbH, Germany). The sintered specimens were cut in the perpendicular direction to the pressing axis, polished, and etched using a CF4 plasma containing 20% O2. The etched and fractured surfaces were sputter coated with gold, and subjected to microstructural characterization using scanning electron microscopy (SEM S4300, Hitachi Ltd., Japan, or FESEM, Quanta 200 FEG, the Netherlands). The micro-hardness of the polished specimens was measured by Vickers indentation (Leica, VMHT (MOT), Germany) at a 1.96 N load for a dwell time of 15 s. Lengths of cracks generated by Vickers
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where P is load in N, c is crack length in m and H is hardness in GPa. E is elastic modulus of the composite in GPa estimated by rule of mixtures using elastic modulii of 440 GPa for SiC and 650 GPa for WC.
$,
*
$ - SiC
*
*
* SiC-50WC
Intensity(a.u.)
indentation (AVK-C2, Akashi Corporation, Japan) at a 196 N load for 30 s were measured and fracture toughness (KIc) in MPa*m1/2 was estimated using the following formula [23]. 1=2 E P ð1Þ K Ic ¼ H c3=2
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$
$
$
$
- WC
$$
*
*
$
*
**
SiC-30WC
SiC-10WC $ $
SiC
$
2.3. Solid particle jet erosion 20
Erosion tests were performed using a jet air erosion machine. Alumina and silicon carbide erodent particles 40–70 mm in size were dried in an oven at 100 1C for 1 h and were mixed with air. The mixed air jet passed through a WC nozzle of 3 mm diameter and were subjected to erode the polished surface of the composite at a stand-off distance of 4 mm. Air pressure was controlled to maintain the particle velocity at 47 m/s with a flow rate of 3 g/min. Erosion tests were done at different angles of impingement: 301, 601, and 901. A schematic diagram of the erosion setup is given in Fig. 1. The weight loss of the sintered ceramics was measured to an accuracy of 70.1 mg after every 1 min of erosion. The test was continued until a steady state weight loss was achieved. The average weight loss in steady state was converted to average volume loss and steady state erosion rate E in mm3/kg was determined as per the following: V ð2Þ E¼ M where V is volume of the material removed in steady state in mm3 and the M is the mass of the erodent particles used in steady state in kg. The erosion results were checked for the reproducibility by conducting whole experiment at least twice for each composition and angle of impingement. Material removal mechanisms were elucidated using SEM-energy dispersive spectroscopy (EDS) analysis of eroded surfaces. 3. Results 3.1. Density and phase analysis Density measurements of sintered specimens are provided in Table 1. Highly dense ( 499% of theoretical density) samples were obtained from sintering in selected hot pressing conditions. The solution-precipitation process caused by the formation of a eutectic liquid between additives (Al2O3–Y2O3) and a native oxide (SiO2) of SiC at sintering temperatures is generally attributed to the densification of SiC ceramics [24]. The sintering additives of Al2O3, Y2O3 and CaO used in the present study also assisted the liquid phase sintering that resulted in dense SiC ceramics and SiC–WC composites. XRD patterns of SiC composites prepared with different WC contents are shown in Fig. 2. When no WC was added, XRD analysis of sintered SiC sample reveals only β-SiC peaks (JCPDS # 01-073-1708), whereas the presence of WC (JCPDS
30
40
50
60
70
80
Angle 2 θ (deg)
Fig. 2. XRD patterns of developed SiC ceramics and SiC–WC composites.
# 00-005-0728) is also observed for the samples prepared using WC. The phases evolved from the contribution of sintering additives are expected to be present on the grain boundaries or grain junctions. However, XRD analysis reveals no other phase on the sintered samples. It is possible that the peak intensity of any other minor phases is out of the detection limit of the XRD instrument. Furthermore, the intensity of WC peaks is increased while that of SiC peaks decreased with an increase in WC content from 10 to 50 wt%. 3.2. Microstructural characterization Figs. 3 and 4, respectively, show the etched and fractured surfaces of SiC ceramics sintered with varying (0, 10, 30, or 50 wt%) content of WC. SiC grains are near equi-axed and no elongated grains are observed in the microstructure of monolithic SiC ceramics (Fig. 3a). This indicates that β-α transformation of SiC did not occur during sintering in the selected hot pressing conditions. WC particles are homogeneously distributed along the grain boundaries and inside the grains of SiC in the SiC–WC composites (Fig. 3b-d). The concentration of WC particles in the composites increased with increasing WC content in the initial powder mixture. Further, the size of the WC particles appearing on the SiC microstructure varied from submicron to micron. An increase in number of large sized WC particles is observed in SiC–50 wt% WC composite microstructure, whereas a uniform distribution of small as well as large sized WC particles can be noted in the case of the SiC–30 wt% WC composite microstructure. In general, the composites show mixed mode of intergranular and transgranular fracture (Fig. 4). However, there appeared to be a change in the extent of transgranular fracture with varying WC content. The transgranular fracture is observed to be dominant in the composite containing 50 wt% WC (see Fig. 4d). Table 2 shows the results obtained from microstructural analysis of surfaces of the sintered specimens. Average SiC grain size decreased from 835 nm for SiC ceramics to 578 nm for SiC–50 wt% WC composite. The average distance between WC particles in sintered composites also
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5 µm
5 µm
5 µm
5 µm
Fig. 3. BSE images of etched surfaces of SiC ceramics prepared with (a) 0% WC, (b) 10% WC, (c) 30% WC, and (d) 50% WC.
5 µm
5 µm
5 µm
5 µm
Fig. 4. SEM images of fractured surfaces of SiC ceramics sintered with (a) 0% WC, (b) 10% WC, (c) 30% WC, and (d) 50% WC.
decreased from 7 to 2 mm with increasing WC content from 10 to 50 wt%. The microstructures of sintered specimens SW10, SW30 and SW50 show 8, 28 and 46 vol% WC contents, respectively.
3.3. Mechanical properties The estimated hardness and fracture toughness values of the investigated composites are listed in Table 3. The hardness was
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Table 2 Microstructural characteristics and mechanical properties of SiC ceramics sintered with 0, 10, 30, and 50 wt% WC. Sample
Grain size (nm)
Inter-particle distance (lm)
Volume fraction
Hardness (GPa)
Fracture toughness (MPa*m1/2)
SW0 SW10 SW30 SW50
8357 73 7247 65 6377 53 5787 62
– 6.80572.763 2.92270.667 2.30370.395
0 0.087 0.01 0.287 0.01 0.467 0.01
23.9570.97 24.0271.12 26.3370.71 24.2671.12
5.857 0.30 6.367 0.22 6.477 0.13 6.667 0.12
Table 3 Erosion rates of SiC–WC composites at normal incidence using alumina and silicon carbide erodent particles. Sample
SW0 SW30 SW50
Erosion rate at normal incidence (mm3/kg) Alumina
Silicon carbide
18.31 6.57 20.49
1044.63 850.73 1223.24
around 24 GPa for SW0, SW10 and SW50, while an increased micro-hardness of 26 GPa is observed for SW30. Indentation fracture toughness of SiC ceramics increased from 5.8 MPa*m1/2 for SW0 to 6.7 MPa*m1/2 for SW50. Fig. 5 shows crack propagation after indenting at a 196 N load in SiC ceramics reinforced with 0 and 30% WC. Only crack deflection is observed in SW0 (Fig. 5(a)), while crack deflection as well as crack bridging by WC occurred in SW30 (Fig. 5(b)). The resistance to crack propagation is observed to increase by additional toughening in the SiC–WC composites. This observation is in accordance with the noted increase in fracture toughness with increasing WC content (see Table 2). 3.4. Erosion results Fig. 6 shows the effect of WC reinforcement and angle of impingement on steady state erosion rate of the sintered SiC composites. Erosion rate of SiC ceramics varied from 240 to 1220 mm3/kg with the WC reinforcement (0, 10, 30 and 50 wt%) and the angle of impingement (301, 601, and 901). The commonly reported erosion rates for SiC ceramics with varying experimental conditions lie in the range of 10–103 mm3/kg [15,25]. Maximum erosion rate occurred at normal impingement for all the SiC–WC composites. The erosion rate of SiC ceramics decreased with WC reinforcement at 601 but the change in erosion rate at 301 and 901 was not regular with WC reinforcement. In general, the erosion rate of all the investigated ceramics increased with increasing angle of impingement from 301 to 901. A similar trend of increasing erosion rate with increasing impact angle is generally reported for brittle materials [26,27]. For WC–Co hard materials, the maximum erosion rate was reported to occur at shallow angles ranging from 501 to 701 [28,29]. 3.5. Worn surface analysis Surfaces of the composites eroded at different angles of impingement (301, 601 or 901) were studied in detail for
elucidating dominant mechanisms of material removal. Typical SEM images of eroded SiC ceramics after erosion at 301 and 901 show grain pull-out and fracture (Fig. 7a and b). The smooth surfaces indicate compacted debris on the eroded surface. Fracture is attributed to the intersection of lateral cracks formed due to the indentation of irregular shaped SiC particles on the brittle SiC ceramics. The shiny edges of the fractured grains, observed in the secondary electron image (see Fig. 7b), essentially indicate lifting of the grains during a sliding/rolling period of the incident particles. This is similar to the observations reported by previous investigators on erosion of silicon carbide and alumina ceramics [12,13,26]. When eroded by alumina particles or silicon carbide particles, extrusion of binder phase and fracture of tungsten carbide grains were observed as dominant mechanisms of material removal in erosion of WC–Co cermets [28,29]. However, SEM images show that the extent of indentationinduced fracture dominates the erosion process of SiC ceramics in the selected erosion conditions. EDS analysis (Fig. 7c) of the SiC ceramics eroded at 901 reveals the presence of Si and C only. The pull-out of WC particles (bright phase) from SiC ceramics containing 10, 30 and 50 wt% WC composites eroded at different angles of impingement can be observed in Figs. 8–10. It is interesting to observe reduced pull-out of WC particles for the SiC–30 wt% WC composite eroded at any impingement angle when compared to other composites (see Fig. 9). This indicates less damage for the SiC–30 wt% WC composite. But, with a further increase in WC content to 50 wt%, the worn surface shown in Fig. 10 reveals increased pull-out of WC particles from the SiC surface. EDS analysis (Fig. 10c) of the worn surface of SiC–50 wt% WC composite after erosion at 901 indicates the presence of Si, W and C. The less damage observed in the SiC–30 wt% WC composite is probably due to (1) the reduced size difference of homogeneously distributed WC particles on its microstructure when compared against that of other composites (refer to Fig. 4) and (2) the superior hardness of the SiC–30 wt% WC composite, compared to other SiC–WC composites (Table 2). This aspect will be discussed later. Table 3 shows steady state erosion rates of SiC–(0, 30 and 50 wt%) WC composites when eroded by SiC or alumina particles at normal incidence. Higher steady state erosion rates are observed for all composites when SiC particles were used. A minimum erosion rate is observed for SiC–30 wt% WC composites, and a maximum is observed for SiC–50 wt% WC. The steady state erosion was also delayed when alumina
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5 µm
5 µm
Fig. 5. Crack propagation in SiC ceramics prepared with (a) 0% WC and (b) 30% WC. The SiC ceramics show crack deflection, whereas SiC–30% WC composite shows crack deflection as well as bridging. 1.6
30
o
60
o
90
4.1. Influence of erodent particle
o
Erosion rate (X 103 mm3/kg)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
10
30
50
WC (wt%) Fig. 6. Steady state erosion rates of SiC–WC composites as function of angle of SiC particle impingement and WC content.
particles were used. For example, the steady state for SiC ceramics started after 20 min when alumina particles were used, whereas 5 min erosion was sufficient to achieve steady state when SiC particles were used. SEM analysis reveals a larger fracture of grains when eroded by harder SiC erodent particles (Fig. 11).
4. Discussion Erosion wear of ceramics is a complex process that generally depends on the material, experimental conditions and/or environmental conditions [27]. In this section, the influence of erodent particles, i.e., SiC and alumina particles, on the erosion behavior of SiC–WC composites will be critically explained. Later, the phenomenal difference in erosion behavior for the investigated SiC ceramic composites will be explained in terms of angle of SiC particle impingement and WC reinforcement. The effect of mechanical properties and microstructural features on erosion wear of the composites will be further highlighted.
When eroded at normal incidence using alumina particles, the present study reveals that the SiC ceramics showed 50–130 times lower erosion wear rate compared to that using SiC particles, irrespective of the WC content (see Table 3). Worn surfaces (Fig. 11) show similar mechanisms of material removal, namely, the fracture of SiC grains for SiC ceramics and WC pull-out and fracture of SiC grains for the SiC–WC composites when impinged by alumina or SiC particles; however, the extent of material degradation was reduced when the alumina erodent was used. This indicates the dominant role of the type of erodent particles on the erosion of the SiC–WC composites. Since softer alumina particles did not form deep indentations in the investigated SiC–WC composites, indentation-induced fracture was suppressed, resulting in small-scale chipping. This is in agreement with the previous observations reported for the erosion of monolithic ceramics by different particles [30–32]. For high impact stresses, plastic flow occurs in either target composites or erodent particles depending upon the geometry and mechanical properties of the target and the particle [33]. The erosion behavior of brittle materials is often related to the ratio of hardness of erodent particles (Hp) to the hardness of the target material (Ht) [30,34]. Hp/Ht ratio in the present study is observed to decrease for any SiC–WC composites when eroded by alumina particles. Lower Hp/Ht ratios in the case of alumina erodent particles indicate less damage of the ceramic composites when compared against SiC erodent particles.
4.2. Influence of impingement angle Brittle materials are subjected to cracking when solid particles impact the surface. The extent of material removal depends on the propagation and intersection of surface and subsurface cracks. For irregular or angular particles indenting on the surface, the intersection of lateral cracks with the surface leads to material removal as chips [11,26]. With an increase in the angle of particle impingement, material removal
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10 µm
10 µm
Fig. 7. (a, b) SEM images of eroded surfaces of SiC–0 wt% WC composite after erosion at 301 and 901. (c) EDS analysis of worn surface shown in (b).
10 µm
10 µm
10 µm
Fig. 8. (a, b, c) SEM images of eroded surfaces of SiC–10 wt% WC composite after erosion by SiC particles at 301, 601 and 901.
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10 µm
10 µm
10 µm
Fig. 9. (a, b, c) SEM images of eroded surfaces of SiC–30 wt% WC composite after erosion by SiC particles at 301, 601 and 901 showing less intergranular fracture and more compaction of debris.
10 µm
10 µm
Fig. 10. (a, b) SEM images of eroded surfaces of SiC–50 wt% WC composite after erosion by SiC particles 301 and 901; (c) represents EDS analysis of worn surface shown in (b).
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(c)
5 µm
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(d)
5 µm
Fig. 11. SEM images of eroded surfaces of SiC–0 and 50 wt% WC composites after erosion at 901 using alumina ((a) and (c), respectively) and silicon carbide ((b) and (d), respectively) erodent.
by lateral cracking increases, reaching a maximum at normal incidence [26]. In the present study, SEM images of eroded surfaces of the SiC ceramics and SiC–WC composites shown in Figs. 7–10 reveal increased fracture of SiC grains when the angle of impingement is increased to 901. WC particles are pulled-out in the case of SiC–WC composites. The contrast difference observed along edges of the fractured regions in worn surfaces of SEM images indicates flow of the fractured grains in the sliding direction of the impacted particles. However, owing to the brittle nature, the erosion of the SiC ceramic composites is said to be dominated by the fracture mechanism. The removal of WC particles indicates the possibility of intergranular as well as transgranular fracture of SiC grains. With increasing angle of impact, the increased normal force leads to increased cracking of grains and higher wear. Results in the present work also show higher wear of SiC ceramics and SiC–WC composites at normal incidence (see Fig. 6). 4.3. Influence of WC reinforcement In the present study, results show that the erosion of SiC ceramics is influenced by the WC content. With 10 wt% WC reinforcement, SiC ceramics showed reduced fracture when compared to that for SiC ceramics with 0 wt% WC. With increasing WC content to 30 wt%, SiC ceramics showed minimal wear with reduced fracture at any angle of impingement. WC
particle removal as well as fracture of SiC grains increased for the SiC–50 wt% WC composite (Fig. 10). This is also in agreement with the erosion rate data provided in Fig. 6. The increase in erosion of SiC–50 wt% WC composites at higher angles of impingement can be attributed to the removal of the available large amount of heavy particles of WC. Since there was no restriction to fracture after removal of WC, SiC grains were probably subjected to increased fracture that led to the increased wear. The homogeneously distributed WC particles throughout the SiC microstructure are attributed to the reduced fracture of SiC grains for SiC–30 wt% WC composite. The difference in erosion behavior of SiC composites prepared with different WC content can be further explained by the mechanical and microstructural characteristics. 4.4. Erosion behavior and mechanical properties The mechanical properties of the SiC–WC composites, particularly fracture toughness and hardness influence the erosion wear behavior. With regards to fracture toughness measurements (see Table 2), SW0 with the lowest fracture toughness of 5.8 MPa*m1/2 showed higher erosion rate, whereas SW30 possessing intermediate fracture toughness of 6.5 MPa*m1/2 exhibited minimum erosion rate. But, SW50 with the highest fracture toughness of 6.7 MPa*m1/2 showed the highest erosion rate at high angles of impingement. Results from the present study and other reported studies for many
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ceramics and ceramic-based composites [35,36] indicate that the local ‘short-crack’ toughness of the materials is important than the bulk or ‘long-crack’ toughness in determining the wear resistance. On relating erosion rate with the hardness data (from Table 2), it is evident that the SW30, possessing a maximum of 26 GPa hardness, exhibited the lowest erosion rate at any investigated angle of impingement, whereas SW50, possessing a lower hardness of 24 GPa, showed the highest erosion rate at a higher angle of impingement (601 and 901). Higher hardness of the composite caused less fracture by indentation of erodent particles. The erosion (E) of brittle target materials with the same erodent and testing parameters can generally be expressed as E ¼ AK Ic m H n
ð3Þ
where A is a proportionality constant, m is 1.3 and n is 0.25 [26]. However, Eq. (3) is not valid with the results obtained from the present erosion study of the SiC–WC composites consisting of different KIc and H values. Several investigations also showed large deviations from the relation shown in Eq. (3), which can be primarily attributed to the fact that these models neglected important contributions of microstructure of the target material in estimating erosion loss [16–18].
content showed superior erosion performance. With 50 wt% WC reinforcement, WC particle pull-out increased and further erosion occurred by subsequent fracture of SiC grains. With increases in angle of impingement of SiC particles, the indentation-induced lateral cracking increased for the SiC– WC composites. The average grain size of SiC, inter-particle distance of WC decreased and fracture toughness increased with increasing WC content to 50 wt%. High hardness and less erosion were obtained with 30 wt% WC content owing to the homogeneously distributed WC particles in the SiC microstructure. Based on the experimental results of improved mechanical and erosion properties, SiC composites with 30 wt% WC reinforcement are safely recommended for nozzles, extrusion dies, and cutting tools.
5. Conclusions Powder compacts consisting of β-SiC, WC (0, 30 and 50 wt%) and sintering additives Y2O3, Al2O3 and CaO were sintered using hot pressing at 1800 1C for 1 h under 40 MPa pressure in an Ar atmosphere to obtain dense SiC–WC composites. Sintered composites were subjected to solid particle erosion using alumina at normal incidence and silicon carbide at 301, 601 and 901. The following major conclusions are drawn:
4.5. Erosion behavior and microstructural features In the present work, microstructural features such as SiC grain size and inter-particle distance of WC particles and volume fraction of the sintered composites were quantified using image analysis. On relating microstructural features from Table 2 with the erosion results shown in Fig. 6, it is evident that the erosion rate decreased with decreasing SiC grain size from 835 nm for SiC ceramics to 640 nm for SiC–30 wt% WC composites. With grain refinement, the hardness increased, leading to a decrease in erosion rate of the composites. However, the erosion rate increased with further decreases in grain size to 580 nm for SiC–50 wt% WC composites. The erosion rate decreased with the decreasing average particle distance between WC particles from 7 mm for SiC–10 wt% WC composites to 3 mm for SiC–30 wt% WC composites. Further decrease in the average inter-particle distance to 2 μm for SiC–50 wt% WC composites caused an increase in erosion rate. These observations indicate that the average grain size and average particle distance of WC have a direct relation to the erosion performance up to 30 wt% WC reinforcement in SiC ceramics. The poor erosion performance with the SiC– 50 wt% WC composites is attributed to the removal of larger amounts (50%) of heavy, large WC particles. In summary, SiC–WC composites exhibited brittle fracture of SiC grains, and WC particle pull-out when eroded by silicon carbide or alumina particles. The erodent particle hardness significantly affected the time required for reaching steady state erosion as well as the extent of material damage for the SiC–WC composites. The extent of damage in selected erosion conditions were also influenced by WC content and angle of impingement. SiC ceramics reinforced with 30 wt% WC
a) SiC and WC evolved as major phases in SiC–WC composites after hot pressing. WC particles were distributed along the grain boundaries and inside the grains of SiC. Mixed mode intergranular and transgranular fractures with increases in the extent of transgranular fracture were observed with increased WC reinforcement. b) Average size of SiC grains decreased from 835 nm for SiC ceramics to 580 nm for SiC–50 wt% WC composite. The average distance between WC particles in sintered composites also decreased from 7 mm to 2 mm with increases in WC content from 10 to 50 wt%. c) A maximum hardness of 26 GPa was obtained for the SiC– 30 wt% WC composite. The indentation fracture toughness increased with WC reinforcement and a maximum fracture toughness of 6.67 MPa*m1/2 was observed for the SiC– 50 wt% WC composite. Crack deflection and crack bridging were major toughening mechanisms for the SiC–WC composites. d) Two orders of magnitude less erosion occurred when SiC–WC composites were eroded at normal incidence by alumina particles when compared against erosion by silicon carbide particles. e) When eroded by silicon carbide particles, erosion rate of the composites increased with increasing angle of impingement from 301 to 901, and decreased with the WC reinforcement up to 30 wt%. A maximum erosion rate of 1223 mm3/kg was noted for SiC–50 wt% WC composites at 901, while a minimum erosion rate of 250–300 mm3/kg was observed for SiC–10 or 30 wt% WC composite at 301. f) Eroded surfaces predominantly showed fracture of SiC grains and pull-out of WC particles. Distribution of WC
S.K. Sharma et al. / Ceramics International 40 (2014) 6829–6839
particles protected the SiC grains from fracture up to 30% WC reinforcement.
Acknowledgment BVMK would like to acknowledge partial support from Faculty Initiation Grant of IIT Roorkee. References [1] T.H. Kosel, Solid particle erosion, in: P.J. Blau (Ed.), Friction, Lubrication and Wear technology, ASM Handbook, vol. 18, ASM International, Ohio, 1992, pp. 199–213. [2] G. Roewer, U. Herzog, K. Trommer, E. Muller, S. Fruhauf, Silicon carbide – a survey of synthetic approaches, properties and applications, Struct. Bond. 101 (2002) 59–135. [3] B.V. Manoj Kumar, Y.W. Kim, D.S. Lim, W.S. Seo, Influence of small amount of sintering additives on unlubricated sliding wear properties of SiC ceramics, Ceram. Int. 37 (8) (2011) 3599–3608. [4] N.P. Padture, In situ-toughened silicon carbide, J. Am. Ceram. Soc. 77 (1994) 519–523. [5] E. Volz, A. Roosen, S.C. Wang, W.C.J. Wei, Formation of intergranular amorphous films during the microstructural development of liquid phase sintered silicon carbide ceramics, J. Mater. Sci. 39 (2004) 4095–5101. [6] S.G. Lee, Y.W. Kim, M. Mitomo, Relationship between microstructure and fracture toughness of toughened silicon carbide ceramics, J. Am. Ceram. Soc. 84 (6) (2001) 1347–1353. [7] Y.W. Kim, M. Mitomo, Fabrication and mechanical properties of silicon carbide–silicon nitride nano composites, J. Mater. Sci. 35 (2000) 5885–5890. [8] G.C. Wei, P.F. Becher, Improvements in mechanical properties in SiC by the addition of TiC particles, J. Am. Ceram. Soc. 67 (1984) 571–574. [9] M.A. Janney, Mechanical properties and oxidation behavior of a hotpressed SiC–15-vol%-TiB2 composite, Am. Ceram. Soc. Bull. 66 (1987) 322–324. [10] E. Medvedovski, Ballistic performance of armour ceramics: Part 1. Influence of design and structure, Ceram. Int. 36 (2010) 2103–2115. [11] A.G. Evans, T.R. Wilshaw, Dynamic solid particle damage in brittle materials, an appraisal, J. Mater. Sci. 12 (1977) 97–116. [12] J.G. Chacon-Nava, F.H. Stott, S.D. de la Torre, A. Martínez-Villafañe, Erosion of Al2O3 and SiC at low impact velocities, Mater. Lett. 55 (2002) 269–273. [13] J.L. Routbort, R.O. Scattergood, A.P.L. Turner, The erosion of reaction bonded SiC, Wear 59 (1980) 363–375. [14] J.L. Routbort, R.O. Scattergood, Anomalous solid-particle erosion rate of hot pressed silicon carbide, J. Am. Ceram. Soc. 63 (1980) 593–595. [15] J.J. Kim, S.K. Park, Solid particle erosion of SiC and SiC–TiB2 composite hot-pressed with Y2O3, Wear 222 (1998) 114–119.
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[16] S.M. Wiederhorn, B.J. Hockey, Effect of material parameters on the erosion resistance of brittle materials, J. Mater. Sci. 18 (1983) 766–780. [17] D.F. Wang, J.H. She, Z.Y. Ma, Effect of microstructure on erosive wear behavior of SiC ceramics, Wear 180 (1995) 35–41. [18] J.L. Routbort, H. Matzke, On the correlation between erosion and fracture parameters in SiC ceramics, J. Mater. Sci. 18 (1983) 1491–1496. [19] D. Jianxin, L. Lili, D. Mingwei, Erosion wear behaviours of SiC/(W,Ti)C laminated ceramic nozzles in dry sand blasting processes, Mater. Sci. Eng. A 444 (2007) 120–129. [20] A.F. Colclough, J.A. Yeomans, Hard particle erosion of silicon carbide and silicon carbide- titanium diboride from room temperature to 1000 1C, Wear 209 (1997) 229–236. [21] J. Pang, J. Li, Low thermal expansion porous SiC–WC composite ceramics, Ceram. Int. 35 (2009) 3517–3520. [22] B.R. Zhang, F. Marino, M. Ferraris, Liquid-phase hot-pressing and WC-particle reinforcement of SiC–Si composites, J. Eur. Ceram. Soc. 14 (6) (1994) 549–555. [23] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, A critical evaluation of indentation techniques for measuring toughness: I. Direct crack measurements, J. Am. Ceram. Soc. 64 (9) (1981) 533–538. [24] L.S. Sigl, H.J. Kleebe, Core/rim structure of liquid-phase-sintered silicon carbide, J. Am. Ceram. Soc. 76 (3) (1993) 773–776. [25] J.L. Routbort, A.P.L. Turner, The erosion rate of reaction bonded SiC containing various amount of free silicon, Wear 84 (1983) 381–385. [26] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, London, 1992. [27] B. Basu, M. Kalin, Tribology of Ceramics and Composites, John Wiley & Sons, Inc., New Jersy, 2011. [28] R.G. Rateick Jr., K.R. Karasek, A.J. Cunningham, K.C. Goretta, J.L. Routbort, Solid particle erosion of tungsten carbide/cobalt cermet and hardened 440C stainless steel – a comparison, Wear 261 (2006) 773–778. [29] U. Beste, L. Hammerstrom, H. Engqvist, S. Rimlinger, S. Jacobson, Particle erosion of cemented carbides with low Co content, Wear 250 (2001) 809–817. [30] P.H. Shipway, I.M. Hutchings, The role of properties in the erosion of brittle materials, Wear 193 (1996) 105–113. [31] Y. Akimune, Y. Katano, M. Matoba, Spherical impact damage and strength degradation in silicon nitrides for automobile turbocharger rotors, J. Am. Ceram. Soc 72 (1989) 1422–1428. [32] M.E. Gulden, Correlation of experimental erosion data with elastic– plastic impact models, J. Am. Ceram. Soc 64 (1981) (C59–C6O). [33] I.M. Hutchings, Ductile-brittle transition and wear maps for the erosion and abrasion of brittle materials, J. Phy. D: Appl. Phys. 25 (1992) A212–A221. [34] S. Srinivasan, R.O. Scattergood, Effect of erodent hardness on erosion of brittle materials, Wear J28 (1988) 139–152. [35] H.H.K. Xu, L. Wei, S. Jahanmir, Grinding force and microcrack density in abrasive machining of silicon nitride, J. Mater. Res. 10 (1995) 3204–3209. [36] C.P. Dogan, J.A. Hawk, Role of zirconia toughening in the abrasive wear of intermetallic and ceramic composites, Wear 212 (1997) 110–118.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6829–6839 www.elsevier.com/locate/ceramint
Erosion behavior of SiC–WC composites Sandan Kumar Sharmaa, B. Venkata Manoj Kumara,n, Kwang-Young Limb, Young-Wook Kimb, S.K. Natha b
a Department of Metallurgical and Materials Engineering, Indian Institute of Technology (IIT) Roorkee, Roorkee, India Functional Ceramics Laboratory, Department of Materials Science and Engineering, the University of Seoul, Seoul, Republic of Korea
Received 6 October 2013; received in revised form 29 November 2013; accepted 30 November 2013 Available online 10 December 2013
Abstract Dense silicon carbide (SiC) ceramics were prepared with 0, 10, 30 or 50 wt% WC particles by hot pressing powder mixtures of SiC, WC and oxide additives at 1800 1C for 1 h under a pressure of 40 MPa in an Ar atmosphere. Effects of alumina or SiC erodent particles and the WC content on the erosion performance of sintered SiC–WC composites were assessed. Microstructures of the sintered composites consisted of WC particles distributed in the equi-axed grain structure of SiC. Fracture surfaces showed a mixed mode of fracture, with a large extent of transgranular fracture observed in SiC ceramics prepared with 30 wt% WC. Crack bridging by WC enhanced toughening of the SiC ceramics. A maximum fracture toughness of 6.7 MPa*m1/2 was observed for the SiC ceramics with 50 wt% WC, whereas a high hardness of 26 GPa was obtained for the SiC ceramics with 30 wt% WC. When eroded at normal incidence, two orders of magnitude less erosion occurred when SiC–WC composites were eroded by alumina particles than that eroded by SiC particles. The erosion rate of the composites increased with increasing angle of SiC particle impingement from 301 to 901, and decreased with WC reinforcement up to 30 wt%. A minimum erosion wear rate of 6.6 mm3/kg was obtained for SiC–30 wt% WC composites. Effects of mechanical properties and microstructure on erosion of the sintered SiC–WC composites are discussed, and the dominant wear mechanisms are also elucidated. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Hot pressing; B. Composites; Silicon carbide; Tungsten carbide; Erosion
1. Introduction Erosion causes major material loss in several structural applications such as gas turbine parts, burner parts, cutting tools, and fluidized bed combustion systems [1]. Owing to their unique combination of superior properties such as high hardness, high temperature strength and excellent resistance to wear and corrosion, silicon carbide (SiC) ceramics are preferred for several tribological applications [2–4]. Liquid phase sintering using different additives like metal oxides, Al–B–C, and AlN–metal oxides has been preferred for the preparation of dense SiC ceramics with a tailored microstructure [4–6]. The mechanical properties were reported to improve by the incorporation of Si3N4 [7], TiC [8] or TiB2 [9] particles in the SiC ceramic matrix. n
Corresponding author. Tel.: þ91 1332 285420; fax: þ 91 1332 285243. E-mail address: [email protected] (B.V.M. Kumar).
Extensive research has been carried out in understanding erosion wear behavior of SiC ceramics and their composites [10–20]. Solid particle erosion of SiC ceramics and their composites is reported to occur generally by brittle fracture as a result of lateral and radial cracking [11–15]. Routbort et al. found that the morphology of the eroded surface of SiC ceramics varied with an increase in the size of the erodent particles [13]. The erosion wear of SiC ceramics is reported to not be in agreement with the predicted erosion wear rates because of the microstructural effects [16–18]. Jianxin et al. [19] studied erosion wear behaviors of SiC/(W,Ti)C laminated ceramic nozzles produced by hot pressing in dry sand blasting processes and reported that the laminated ceramic nozzles were superior to the homologous stress-free ceramic nozzle due to the formation of compressive residual stresses in the nozzle entry region. Kim and Park [15] reported that the erosion rates of hot-pressed monolithic SiC ceramics and SiC–TiB2 composites did not increase monotonically with increasing particle
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.146
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Table 1 Sample designation, batch composition, and sintered density of the investigated powder mixtures. Sample designation
SW0 SW10 SW30 SW50
Sintered density (g/cc)
Batch composition (wt%) β-SiC
α-SiC
WC
Al2O3
Y2O3
CaO
94.05 84.15 64.35 44.55
0.95 0.85 0.65 0.45
0 10 30 50
3.5 3.5 3.5 3.5
1.0 1.0 1.0 1.0
0.879 0.879 0.879 0.879
size. Rather, the erosion rate was higher for the SiC–TiB2 composite with higher fracture toughness but had a lower hardness compared to monolithic SiC, indicating that the erosion of these materials is largely controlled by the plastic deformation [15]. Large erodent particle size and elevated temperatures are reported to lead to lower erosion rates for SiC–TiB2 composites when compared against SiC ceramics [20]. A new combination that has received much less attention is the incorporation of tungsten carbide (WC) in SiC ceramics. Improved wear resistance is expected by adding tough and strong WC in the hard SiC matrix. Powder metallurgy route is suitable for manufacturing such composites. Pang and Li prepared porous SiC–WC composites by solid reaction of Si and WC for applications that involve low thermal expansion [21]. Zhang et al. prepared dense and strong SiC–Si composites reinforced by WC particle by hot pressing [22]. To the best of our knowledge, no report is available in the open literature on hot pressing of SiC and WC powders to prepare dense SiC–WC composites for their use in tribological applications. In the present work, SiC ceramic composites with varying WC content (0–50 wt%) were prepared by hot pressing technique. The erosion behavior of the sintered composites was estimated using alumina and SiC particles at normal incidence. Effects of angle of impingement (301, 601 and 901) of SiC particles and WC reinforcement on the erosion performance were particularly assessed. The dominant material removal mechanisms in the selected erosion conditions were elucidated.
2. Experimental procedure 2.1. Material preparation Commercially available β-SiC (BF-17, Hermann C. Starck, Germany), α-SiC (A1, Showa Denko, Japan), WC (High Purity Chemicals, Saitama Pre., Japan), metal oxides Al2O3, (AKP30, Sumitomo Chemical Co., Tokyo, Japan), Y2O3, (High Purity Chemicals, Saitama Pre., Japan) and CaO (High Purity Chemicals, Saitama Pre., Japan) were used as starting powders. Powder mixtures containing 0, 30, and 50 wt% WC were prepared. The respective powder batches were milled in ethanol for 24 h using SiC grinding balls. The milled slurry was dried, sieved, and hot-pressed at 1800 1C for 1 h under
3.23 3.51 4.31 5.45
Fig. 1. Schematic diagram of erosion set-up.
a pressure of 40 MPa in Ar atmosphere to obtain sintered samples of 30 mm in diameter and 4 mm in thickness. The details of sample designation, batch composition and additive systems are given in Table 1. 2.2. Characterization of sintered composites The volume and weight of the sintered specimens were measured and the bulk density was estimated. For calculating theoretical densities by rule of mixtures, the following theoretical densities (in g/cc) of each constituent are considered: β-SiC 3.216, WC 15.669, Al2O3 3.987, Y2O3 5.032, and CaO 3.374. Phase analysis of the sintered specimens was performed using X-ray diffractometry (XRD, D8 Discover, Bruker AXS GmbH, Germany). The sintered specimens were cut in the perpendicular direction to the pressing axis, polished, and etched using a CF4 plasma containing 20% O2. The etched and fractured surfaces were sputter coated with gold, and subjected to microstructural characterization using scanning electron microscopy (SEM S4300, Hitachi Ltd., Japan, or FESEM, Quanta 200 FEG, the Netherlands). The micro-hardness of the polished specimens was measured by Vickers indentation (Leica, VMHT (MOT), Germany) at a 1.96 N load for a dwell time of 15 s. Lengths of cracks generated by Vickers
S.K. Sharma et al. / Ceramics International 40 (2014) 6829–6839
where P is load in N, c is crack length in m and H is hardness in GPa. E is elastic modulus of the composite in GPa estimated by rule of mixtures using elastic modulii of 440 GPa for SiC and 650 GPa for WC.
$,
*
$ - SiC
*
*
* SiC-50WC
Intensity(a.u.)
indentation (AVK-C2, Akashi Corporation, Japan) at a 196 N load for 30 s were measured and fracture toughness (KIc) in MPa*m1/2 was estimated using the following formula [23]. 1=2 E P ð1Þ K Ic ¼ H c3=2
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$
$
$
$
- WC
$$
*
*
$
*
**
SiC-30WC
SiC-10WC $ $
SiC
$
2.3. Solid particle jet erosion 20
Erosion tests were performed using a jet air erosion machine. Alumina and silicon carbide erodent particles 40–70 mm in size were dried in an oven at 100 1C for 1 h and were mixed with air. The mixed air jet passed through a WC nozzle of 3 mm diameter and were subjected to erode the polished surface of the composite at a stand-off distance of 4 mm. Air pressure was controlled to maintain the particle velocity at 47 m/s with a flow rate of 3 g/min. Erosion tests were done at different angles of impingement: 301, 601, and 901. A schematic diagram of the erosion setup is given in Fig. 1. The weight loss of the sintered ceramics was measured to an accuracy of 70.1 mg after every 1 min of erosion. The test was continued until a steady state weight loss was achieved. The average weight loss in steady state was converted to average volume loss and steady state erosion rate E in mm3/kg was determined as per the following: V ð2Þ E¼ M where V is volume of the material removed in steady state in mm3 and the M is the mass of the erodent particles used in steady state in kg. The erosion results were checked for the reproducibility by conducting whole experiment at least twice for each composition and angle of impingement. Material removal mechanisms were elucidated using SEM-energy dispersive spectroscopy (EDS) analysis of eroded surfaces. 3. Results 3.1. Density and phase analysis Density measurements of sintered specimens are provided in Table 1. Highly dense ( 499% of theoretical density) samples were obtained from sintering in selected hot pressing conditions. The solution-precipitation process caused by the formation of a eutectic liquid between additives (Al2O3–Y2O3) and a native oxide (SiO2) of SiC at sintering temperatures is generally attributed to the densification of SiC ceramics [24]. The sintering additives of Al2O3, Y2O3 and CaO used in the present study also assisted the liquid phase sintering that resulted in dense SiC ceramics and SiC–WC composites. XRD patterns of SiC composites prepared with different WC contents are shown in Fig. 2. When no WC was added, XRD analysis of sintered SiC sample reveals only β-SiC peaks (JCPDS # 01-073-1708), whereas the presence of WC (JCPDS
30
40
50
60
70
80
Angle 2 θ (deg)
Fig. 2. XRD patterns of developed SiC ceramics and SiC–WC composites.
# 00-005-0728) is also observed for the samples prepared using WC. The phases evolved from the contribution of sintering additives are expected to be present on the grain boundaries or grain junctions. However, XRD analysis reveals no other phase on the sintered samples. It is possible that the peak intensity of any other minor phases is out of the detection limit of the XRD instrument. Furthermore, the intensity of WC peaks is increased while that of SiC peaks decreased with an increase in WC content from 10 to 50 wt%. 3.2. Microstructural characterization Figs. 3 and 4, respectively, show the etched and fractured surfaces of SiC ceramics sintered with varying (0, 10, 30, or 50 wt%) content of WC. SiC grains are near equi-axed and no elongated grains are observed in the microstructure of monolithic SiC ceramics (Fig. 3a). This indicates that β-α transformation of SiC did not occur during sintering in the selected hot pressing conditions. WC particles are homogeneously distributed along the grain boundaries and inside the grains of SiC in the SiC–WC composites (Fig. 3b-d). The concentration of WC particles in the composites increased with increasing WC content in the initial powder mixture. Further, the size of the WC particles appearing on the SiC microstructure varied from submicron to micron. An increase in number of large sized WC particles is observed in SiC–50 wt% WC composite microstructure, whereas a uniform distribution of small as well as large sized WC particles can be noted in the case of the SiC–30 wt% WC composite microstructure. In general, the composites show mixed mode of intergranular and transgranular fracture (Fig. 4). However, there appeared to be a change in the extent of transgranular fracture with varying WC content. The transgranular fracture is observed to be dominant in the composite containing 50 wt% WC (see Fig. 4d). Table 2 shows the results obtained from microstructural analysis of surfaces of the sintered specimens. Average SiC grain size decreased from 835 nm for SiC ceramics to 578 nm for SiC–50 wt% WC composite. The average distance between WC particles in sintered composites also
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5 µm
5 µm
5 µm
5 µm
Fig. 3. BSE images of etched surfaces of SiC ceramics prepared with (a) 0% WC, (b) 10% WC, (c) 30% WC, and (d) 50% WC.
5 µm
5 µm
5 µm
5 µm
Fig. 4. SEM images of fractured surfaces of SiC ceramics sintered with (a) 0% WC, (b) 10% WC, (c) 30% WC, and (d) 50% WC.
decreased from 7 to 2 mm with increasing WC content from 10 to 50 wt%. The microstructures of sintered specimens SW10, SW30 and SW50 show 8, 28 and 46 vol% WC contents, respectively.
3.3. Mechanical properties The estimated hardness and fracture toughness values of the investigated composites are listed in Table 3. The hardness was
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Table 2 Microstructural characteristics and mechanical properties of SiC ceramics sintered with 0, 10, 30, and 50 wt% WC. Sample
Grain size (nm)
Inter-particle distance (lm)
Volume fraction
Hardness (GPa)
Fracture toughness (MPa*m1/2)
SW0 SW10 SW30 SW50
8357 73 7247 65 6377 53 5787 62
– 6.80572.763 2.92270.667 2.30370.395
0 0.087 0.01 0.287 0.01 0.467 0.01
23.9570.97 24.0271.12 26.3370.71 24.2671.12
5.857 0.30 6.367 0.22 6.477 0.13 6.667 0.12
Table 3 Erosion rates of SiC–WC composites at normal incidence using alumina and silicon carbide erodent particles. Sample
SW0 SW30 SW50
Erosion rate at normal incidence (mm3/kg) Alumina
Silicon carbide
18.31 6.57 20.49
1044.63 850.73 1223.24
around 24 GPa for SW0, SW10 and SW50, while an increased micro-hardness of 26 GPa is observed for SW30. Indentation fracture toughness of SiC ceramics increased from 5.8 MPa*m1/2 for SW0 to 6.7 MPa*m1/2 for SW50. Fig. 5 shows crack propagation after indenting at a 196 N load in SiC ceramics reinforced with 0 and 30% WC. Only crack deflection is observed in SW0 (Fig. 5(a)), while crack deflection as well as crack bridging by WC occurred in SW30 (Fig. 5(b)). The resistance to crack propagation is observed to increase by additional toughening in the SiC–WC composites. This observation is in accordance with the noted increase in fracture toughness with increasing WC content (see Table 2). 3.4. Erosion results Fig. 6 shows the effect of WC reinforcement and angle of impingement on steady state erosion rate of the sintered SiC composites. Erosion rate of SiC ceramics varied from 240 to 1220 mm3/kg with the WC reinforcement (0, 10, 30 and 50 wt%) and the angle of impingement (301, 601, and 901). The commonly reported erosion rates for SiC ceramics with varying experimental conditions lie in the range of 10–103 mm3/kg [15,25]. Maximum erosion rate occurred at normal impingement for all the SiC–WC composites. The erosion rate of SiC ceramics decreased with WC reinforcement at 601 but the change in erosion rate at 301 and 901 was not regular with WC reinforcement. In general, the erosion rate of all the investigated ceramics increased with increasing angle of impingement from 301 to 901. A similar trend of increasing erosion rate with increasing impact angle is generally reported for brittle materials [26,27]. For WC–Co hard materials, the maximum erosion rate was reported to occur at shallow angles ranging from 501 to 701 [28,29]. 3.5. Worn surface analysis Surfaces of the composites eroded at different angles of impingement (301, 601 or 901) were studied in detail for
elucidating dominant mechanisms of material removal. Typical SEM images of eroded SiC ceramics after erosion at 301 and 901 show grain pull-out and fracture (Fig. 7a and b). The smooth surfaces indicate compacted debris on the eroded surface. Fracture is attributed to the intersection of lateral cracks formed due to the indentation of irregular shaped SiC particles on the brittle SiC ceramics. The shiny edges of the fractured grains, observed in the secondary electron image (see Fig. 7b), essentially indicate lifting of the grains during a sliding/rolling period of the incident particles. This is similar to the observations reported by previous investigators on erosion of silicon carbide and alumina ceramics [12,13,26]. When eroded by alumina particles or silicon carbide particles, extrusion of binder phase and fracture of tungsten carbide grains were observed as dominant mechanisms of material removal in erosion of WC–Co cermets [28,29]. However, SEM images show that the extent of indentationinduced fracture dominates the erosion process of SiC ceramics in the selected erosion conditions. EDS analysis (Fig. 7c) of the SiC ceramics eroded at 901 reveals the presence of Si and C only. The pull-out of WC particles (bright phase) from SiC ceramics containing 10, 30 and 50 wt% WC composites eroded at different angles of impingement can be observed in Figs. 8–10. It is interesting to observe reduced pull-out of WC particles for the SiC–30 wt% WC composite eroded at any impingement angle when compared to other composites (see Fig. 9). This indicates less damage for the SiC–30 wt% WC composite. But, with a further increase in WC content to 50 wt%, the worn surface shown in Fig. 10 reveals increased pull-out of WC particles from the SiC surface. EDS analysis (Fig. 10c) of the worn surface of SiC–50 wt% WC composite after erosion at 901 indicates the presence of Si, W and C. The less damage observed in the SiC–30 wt% WC composite is probably due to (1) the reduced size difference of homogeneously distributed WC particles on its microstructure when compared against that of other composites (refer to Fig. 4) and (2) the superior hardness of the SiC–30 wt% WC composite, compared to other SiC–WC composites (Table 2). This aspect will be discussed later. Table 3 shows steady state erosion rates of SiC–(0, 30 and 50 wt%) WC composites when eroded by SiC or alumina particles at normal incidence. Higher steady state erosion rates are observed for all composites when SiC particles were used. A minimum erosion rate is observed for SiC–30 wt% WC composites, and a maximum is observed for SiC–50 wt% WC. The steady state erosion was also delayed when alumina
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5 µm
5 µm
Fig. 5. Crack propagation in SiC ceramics prepared with (a) 0% WC and (b) 30% WC. The SiC ceramics show crack deflection, whereas SiC–30% WC composite shows crack deflection as well as bridging. 1.6
30
o
60
o
90
4.1. Influence of erodent particle
o
Erosion rate (X 103 mm3/kg)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
10
30
50
WC (wt%) Fig. 6. Steady state erosion rates of SiC–WC composites as function of angle of SiC particle impingement and WC content.
particles were used. For example, the steady state for SiC ceramics started after 20 min when alumina particles were used, whereas 5 min erosion was sufficient to achieve steady state when SiC particles were used. SEM analysis reveals a larger fracture of grains when eroded by harder SiC erodent particles (Fig. 11).
4. Discussion Erosion wear of ceramics is a complex process that generally depends on the material, experimental conditions and/or environmental conditions [27]. In this section, the influence of erodent particles, i.e., SiC and alumina particles, on the erosion behavior of SiC–WC composites will be critically explained. Later, the phenomenal difference in erosion behavior for the investigated SiC ceramic composites will be explained in terms of angle of SiC particle impingement and WC reinforcement. The effect of mechanical properties and microstructural features on erosion wear of the composites will be further highlighted.
When eroded at normal incidence using alumina particles, the present study reveals that the SiC ceramics showed 50–130 times lower erosion wear rate compared to that using SiC particles, irrespective of the WC content (see Table 3). Worn surfaces (Fig. 11) show similar mechanisms of material removal, namely, the fracture of SiC grains for SiC ceramics and WC pull-out and fracture of SiC grains for the SiC–WC composites when impinged by alumina or SiC particles; however, the extent of material degradation was reduced when the alumina erodent was used. This indicates the dominant role of the type of erodent particles on the erosion of the SiC–WC composites. Since softer alumina particles did not form deep indentations in the investigated SiC–WC composites, indentation-induced fracture was suppressed, resulting in small-scale chipping. This is in agreement with the previous observations reported for the erosion of monolithic ceramics by different particles [30–32]. For high impact stresses, plastic flow occurs in either target composites or erodent particles depending upon the geometry and mechanical properties of the target and the particle [33]. The erosion behavior of brittle materials is often related to the ratio of hardness of erodent particles (Hp) to the hardness of the target material (Ht) [30,34]. Hp/Ht ratio in the present study is observed to decrease for any SiC–WC composites when eroded by alumina particles. Lower Hp/Ht ratios in the case of alumina erodent particles indicate less damage of the ceramic composites when compared against SiC erodent particles.
4.2. Influence of impingement angle Brittle materials are subjected to cracking when solid particles impact the surface. The extent of material removal depends on the propagation and intersection of surface and subsurface cracks. For irregular or angular particles indenting on the surface, the intersection of lateral cracks with the surface leads to material removal as chips [11,26]. With an increase in the angle of particle impingement, material removal
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10 µm
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Fig. 7. (a, b) SEM images of eroded surfaces of SiC–0 wt% WC composite after erosion at 301 and 901. (c) EDS analysis of worn surface shown in (b).
10 µm
10 µm
10 µm
Fig. 8. (a, b, c) SEM images of eroded surfaces of SiC–10 wt% WC composite after erosion by SiC particles at 301, 601 and 901.
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10 µm
10 µm
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Fig. 9. (a, b, c) SEM images of eroded surfaces of SiC–30 wt% WC composite after erosion by SiC particles at 301, 601 and 901 showing less intergranular fracture and more compaction of debris.
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Fig. 10. (a, b) SEM images of eroded surfaces of SiC–50 wt% WC composite after erosion by SiC particles 301 and 901; (c) represents EDS analysis of worn surface shown in (b).
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(c)
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5 µm
(d)
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Fig. 11. SEM images of eroded surfaces of SiC–0 and 50 wt% WC composites after erosion at 901 using alumina ((a) and (c), respectively) and silicon carbide ((b) and (d), respectively) erodent.
by lateral cracking increases, reaching a maximum at normal incidence [26]. In the present study, SEM images of eroded surfaces of the SiC ceramics and SiC–WC composites shown in Figs. 7–10 reveal increased fracture of SiC grains when the angle of impingement is increased to 901. WC particles are pulled-out in the case of SiC–WC composites. The contrast difference observed along edges of the fractured regions in worn surfaces of SEM images indicates flow of the fractured grains in the sliding direction of the impacted particles. However, owing to the brittle nature, the erosion of the SiC ceramic composites is said to be dominated by the fracture mechanism. The removal of WC particles indicates the possibility of intergranular as well as transgranular fracture of SiC grains. With increasing angle of impact, the increased normal force leads to increased cracking of grains and higher wear. Results in the present work also show higher wear of SiC ceramics and SiC–WC composites at normal incidence (see Fig. 6). 4.3. Influence of WC reinforcement In the present study, results show that the erosion of SiC ceramics is influenced by the WC content. With 10 wt% WC reinforcement, SiC ceramics showed reduced fracture when compared to that for SiC ceramics with 0 wt% WC. With increasing WC content to 30 wt%, SiC ceramics showed minimal wear with reduced fracture at any angle of impingement. WC
particle removal as well as fracture of SiC grains increased for the SiC–50 wt% WC composite (Fig. 10). This is also in agreement with the erosion rate data provided in Fig. 6. The increase in erosion of SiC–50 wt% WC composites at higher angles of impingement can be attributed to the removal of the available large amount of heavy particles of WC. Since there was no restriction to fracture after removal of WC, SiC grains were probably subjected to increased fracture that led to the increased wear. The homogeneously distributed WC particles throughout the SiC microstructure are attributed to the reduced fracture of SiC grains for SiC–30 wt% WC composite. The difference in erosion behavior of SiC composites prepared with different WC content can be further explained by the mechanical and microstructural characteristics. 4.4. Erosion behavior and mechanical properties The mechanical properties of the SiC–WC composites, particularly fracture toughness and hardness influence the erosion wear behavior. With regards to fracture toughness measurements (see Table 2), SW0 with the lowest fracture toughness of 5.8 MPa*m1/2 showed higher erosion rate, whereas SW30 possessing intermediate fracture toughness of 6.5 MPa*m1/2 exhibited minimum erosion rate. But, SW50 with the highest fracture toughness of 6.7 MPa*m1/2 showed the highest erosion rate at high angles of impingement. Results from the present study and other reported studies for many
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ceramics and ceramic-based composites [35,36] indicate that the local ‘short-crack’ toughness of the materials is important than the bulk or ‘long-crack’ toughness in determining the wear resistance. On relating erosion rate with the hardness data (from Table 2), it is evident that the SW30, possessing a maximum of 26 GPa hardness, exhibited the lowest erosion rate at any investigated angle of impingement, whereas SW50, possessing a lower hardness of 24 GPa, showed the highest erosion rate at a higher angle of impingement (601 and 901). Higher hardness of the composite caused less fracture by indentation of erodent particles. The erosion (E) of brittle target materials with the same erodent and testing parameters can generally be expressed as E ¼ AK Ic m H n
ð3Þ
where A is a proportionality constant, m is 1.3 and n is 0.25 [26]. However, Eq. (3) is not valid with the results obtained from the present erosion study of the SiC–WC composites consisting of different KIc and H values. Several investigations also showed large deviations from the relation shown in Eq. (3), which can be primarily attributed to the fact that these models neglected important contributions of microstructure of the target material in estimating erosion loss [16–18].
content showed superior erosion performance. With 50 wt% WC reinforcement, WC particle pull-out increased and further erosion occurred by subsequent fracture of SiC grains. With increases in angle of impingement of SiC particles, the indentation-induced lateral cracking increased for the SiC– WC composites. The average grain size of SiC, inter-particle distance of WC decreased and fracture toughness increased with increasing WC content to 50 wt%. High hardness and less erosion were obtained with 30 wt% WC content owing to the homogeneously distributed WC particles in the SiC microstructure. Based on the experimental results of improved mechanical and erosion properties, SiC composites with 30 wt% WC reinforcement are safely recommended for nozzles, extrusion dies, and cutting tools.
5. Conclusions Powder compacts consisting of β-SiC, WC (0, 30 and 50 wt%) and sintering additives Y2O3, Al2O3 and CaO were sintered using hot pressing at 1800 1C for 1 h under 40 MPa pressure in an Ar atmosphere to obtain dense SiC–WC composites. Sintered composites were subjected to solid particle erosion using alumina at normal incidence and silicon carbide at 301, 601 and 901. The following major conclusions are drawn:
4.5. Erosion behavior and microstructural features In the present work, microstructural features such as SiC grain size and inter-particle distance of WC particles and volume fraction of the sintered composites were quantified using image analysis. On relating microstructural features from Table 2 with the erosion results shown in Fig. 6, it is evident that the erosion rate decreased with decreasing SiC grain size from 835 nm for SiC ceramics to 640 nm for SiC–30 wt% WC composites. With grain refinement, the hardness increased, leading to a decrease in erosion rate of the composites. However, the erosion rate increased with further decreases in grain size to 580 nm for SiC–50 wt% WC composites. The erosion rate decreased with the decreasing average particle distance between WC particles from 7 mm for SiC–10 wt% WC composites to 3 mm for SiC–30 wt% WC composites. Further decrease in the average inter-particle distance to 2 μm for SiC–50 wt% WC composites caused an increase in erosion rate. These observations indicate that the average grain size and average particle distance of WC have a direct relation to the erosion performance up to 30 wt% WC reinforcement in SiC ceramics. The poor erosion performance with the SiC– 50 wt% WC composites is attributed to the removal of larger amounts (50%) of heavy, large WC particles. In summary, SiC–WC composites exhibited brittle fracture of SiC grains, and WC particle pull-out when eroded by silicon carbide or alumina particles. The erodent particle hardness significantly affected the time required for reaching steady state erosion as well as the extent of material damage for the SiC–WC composites. The extent of damage in selected erosion conditions were also influenced by WC content and angle of impingement. SiC ceramics reinforced with 30 wt% WC
a) SiC and WC evolved as major phases in SiC–WC composites after hot pressing. WC particles were distributed along the grain boundaries and inside the grains of SiC. Mixed mode intergranular and transgranular fractures with increases in the extent of transgranular fracture were observed with increased WC reinforcement. b) Average size of SiC grains decreased from 835 nm for SiC ceramics to 580 nm for SiC–50 wt% WC composite. The average distance between WC particles in sintered composites also decreased from 7 mm to 2 mm with increases in WC content from 10 to 50 wt%. c) A maximum hardness of 26 GPa was obtained for the SiC– 30 wt% WC composite. The indentation fracture toughness increased with WC reinforcement and a maximum fracture toughness of 6.67 MPa*m1/2 was observed for the SiC– 50 wt% WC composite. Crack deflection and crack bridging were major toughening mechanisms for the SiC–WC composites. d) Two orders of magnitude less erosion occurred when SiC–WC composites were eroded at normal incidence by alumina particles when compared against erosion by silicon carbide particles. e) When eroded by silicon carbide particles, erosion rate of the composites increased with increasing angle of impingement from 301 to 901, and decreased with the WC reinforcement up to 30 wt%. A maximum erosion rate of 1223 mm3/kg was noted for SiC–50 wt% WC composites at 901, while a minimum erosion rate of 250–300 mm3/kg was observed for SiC–10 or 30 wt% WC composite at 301. f) Eroded surfaces predominantly showed fracture of SiC grains and pull-out of WC particles. Distribution of WC
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particles protected the SiC grains from fracture up to 30% WC reinforcement.
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